Preheat the oven to 400F.

Add the cauliflower to a saucepan, cover with water, and bring to the boil. Cook until tender, about 10 minutes. Alternatively, cook the cauliflower in the microwave.

Drain the cauliflower and transfer it to a food processor, together with the melted butter, sour cream, salt and pepper. Blend until smooth. 

Transfer the mashed cauliflower to a baking dish, then sprinkle the cheese and bacon bits over the top. Roast until the cheese has melted and has turned golden brown. 

Garnish with green onions, then serve.